They didn't "date" at all. Working together, sharing a brief kiss during the final mission, and *promising* a date that never comes to pass is not "dating." The full extent of Steve's "relationship" with Peggy is What Might Have Been. That's it.

And *everything* changes based on Cap's extraordinarily unique sci-fi situation. 70 years and 2-3 generations passed him by in an instant, and he's essentially traveled through time to the future. Whatever feelings he still has for Peggy are meaningless and moot, since she's already been dead and gone to him for *70 years.*

They were in the middle of the war and it was a different time period for relationship, but it's clear in the film there are strong feelings between Cap and Peggy, it's not just some idle crush. Regardless though, to then hook up with your first loves granddaughter is weird and creepy, no matter how many excuses you make.

__________________"Now this life is etched in black but I wont be looking back, the rain washed out the tracks, I'll never find again"